System and method for providing accurate position location information to military forces in a disadvantaged signal environment

ABSTRACT

A system for determining a location in a disadvantaged signal environment includes three aerial vehicles hovering at high altitude and spaced apart to form a triangle, and a mother aerial vehicle positioned a distance away and at a lower altitude. The mother aerial vehicle acquires and transmits coarse geolocation information, using a pulse compression, high-power X Band radar and directional antenna, to each of the three aerial vehicles to direct them to coarse geo-positions above designated respective ground locations. One of the three aerial vehicles has a synthetic aperture radar for producing a terrain strip-map that is mensurated against a map database to provide fine position adjustments for each of the three aerial vehicles, which are also also configured to transmit a respective signal coded with its latitude, longitude, and altitude, for a computing device to perform time difference of arrival measurements of the signals to determine its location.

CROSS REFERENCES TO RELATED APPLICATIONS

This application claims priority on U.S. Provisional Application Ser.No. 62/444,905, filed on Jan. 11, 2017, the disclosures of which areincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to providing and/or obtaining positioninformation in a hostile environment, and more particularly relates to aposition, navigation, and timing system that is highly jam-resistant andspoof-resistant.

BACKGROUND OF THE INVENTION

The availability of accurate positional information is often crucial forconducting military operations and other covert operations. Theavailability of GPS signals enable the use of smart bombs, and advancednavigation systems. The challenges of conducting operations in aGPS-denied or GPS-spoofed territory persist, and pose a serious threatto the success of future missions—threats that can result inconsequential loss to life and property. In the case of GPS-spoofing, ithas already been utilized to command an air vehicle to “safe-land” inhostile territory as a result of reliance on erroneous GPS data. In2011, the nation of Iran captured a batwing stealth drone by spoofingthe GPS signals that it received, tricking the drone into landing atwhat it believed was its home base, but was actually a location wellwithin Iran's borders.

Early in 2016 the U.S. Air Force and the Navy each requested proposalsfor back-up technology that would supplement or supplant the use of GPSin the instance where a similar spoofing, or an outright denial of GPSsignals may be experienced.

The present invention is directed to such a system that overcomesproblems that may be experienced with position, navigation, and timingsystems.

Some early position, navigation, and/or position/navigation-relatedtechnology is shown by the following U.S. Patent and Patent ApplicationPublications: U.S. Pat. No. 2,470,787 to Nosker; U.S. Pat. No. 3,384,891to Andersen; U.S. Pat. No. 3,430,234 to Wright; U.S. Pat. No. 3,471,856to Laughlin; U.S. Pat. No. 3,495,260 to Laughlin; U.S. Pat. No.3,554,995 to Bottenberg; U.S. Pat. No. 3,611,379 to Deckett; U.S. Pat.No. 3,705,404 to Chisholm; U.S. Pat. No. 3,742,498 to Dunn; U.S. Pat.No. 3,789,409 to Easton; U.S. Pat. No. 3,810,179 to Merrick; U.S. Pat.No. 3,836,970 to Reitzig; U.S. Pat. No. 3,852,750 to Klein; U.S. Pat.No. 3,886,553 to Bates; U.S. Pat. No. 3,988,734 to Elwood; U.S. Pat. No.4,114,155 to Raab; U.S. Pat. No. 4,161,730 to Anderson; U.S. Pat. No.4,179,693 to Evans; U.S. Pat. No. 4,253,098 to Blythe; U.S. Pat. No.4,359,733 to O'Neill; U.S. Pat. No. 4,386,355 to Drew; U.S. Pat. No.4,472,720 to Reesor; U.S. Pat. No. 4,839,656 to O'Neill; U.S. Pat. No.4,987,420 to Inamiya; U.S. Pat. No. 5,014,066 to Counselman; U.S. Pat.No. 5,017,926 to Ames; U.S. Pat. No. 5,432,520 to Schneider; U.S. Pat.No. 5,521,817 to Burdoin; U.S. Pat. No. 5,944,770 to Enge; U.S. Pat. No.5,999,124 to Sheynblat; U.S. Pat. No. 5,999,129 to Rose; U.S. Pat. No.6,167,263 to Campbell; U.S. Pat. No. 6,249,252 to Dupray; U.S. Pat. No.6,407,703 to Minter; U.S. Pat. No. 6,529,820 to Tomescu; U.S. Pat. No.6,618,016 to Hannan; U.S. Pat. No. 6,785,553 to Chang; U.S. Pat. No.6,819,291 to Lackey; U.S. Pat. No. 6,911,936 to Stayton; U.S. Pat. No.6,961,019 to McConnell; U.S. Pat. No. 7,043,355 to Lai; 2007/0001898 toTwitchell; 2007/0222665 to Koeneman; U.S. Pat. No. 7,292,935 to Yoon;2008/0042901 to Smith; 2008/0158059 to Bull; U.S. Pat. No. 7,460,870 toMoeglein; U.S. Pat. No. 7,711,476 to Chiou; U.S. Pat. No. 7,904,244 toSugla; U.S. Pat. No. 7,990,314 to Liao; U.S. Pat. No. 8,086,351 toGaudiano; U.S. Pat. No. 8,155,666 to Alizadeh-Shadbiz U.S. Pat. No.8,922,421 to Pomietlasz; and U.S. Pat. No. 9,218,741 to Wu.

OBJECTS OF THE INVENTION

It is an object of the invention to provide apparatus that permitsSpecial Operations Forces (SOF) and other users to rapidly determine ifGPS spoofing is occurring.

It is a further object of the invention to provide a system thatcircumvents GPS spooling or outright denial of GPS signals in a hostileterritory.

It is another object of the invention to provide a system thatsupplements or replaces the use of GPS in the instance where spoofing oroutright denial of GPS signals is experienced in a hostile territory.

Further objects and advantages of the invention will become apparentfrom the following description and claims, and from the accompanyingdrawings.

SUMMARY OF THE INVENTION

This Summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This Summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used to limit the scope of the claimed subject matter.

In accordance with at least one embodiment of the present invention, asystem for determining a location in a disadvantaged signal environmentmay include a plurality of aerial vehicles at a first altitude and beingspaced apart between five to fifty miles, and typically being spaced atleast twenty-five miles apart, and may be uniformly space apart, andwith a mother aerial vehicle positioned a distance away from theplurality of aerial vehicles at a second altitude. The mother aerialvehicle, such as the radar-equipped Navy MH-60R ASW Helicopter, maytypically be positioned at an altitude of less than 10,000 feet AGL,while the plurality of aerial vehicles are each typically positioned ata much higher altitude, to be above adverse weather, and above a rangecapability of anti-aircraft artillery guns in the area.

The plurality of aerial vehicles may each be configured to hover, andmay preferably be spaced apart to form a triangle. The mother aerialvehicle may be configured to acquire and transmit coarse geolocationinformation to each of the plurality of aerial vehicles to direct eachof the plurality of aerial vehicles to coarse geo-positions above adesignated respective ground location. The mother aerial vehicle may beequipped with a pulse compression, high-power X Band radar and adirectional antenna that may be used to transmit the coarse geolocationinformation to each of the plurality of aerial vehicles using an X bandTDL or TCDL link, with a beam width of one degree and side lobe levelsof less than 40 dB. The X band radar may have a bearing accuracy of 0.2degrees rms and an rms range measurement accuracy of less than twohundred feet, for coarsely locating the UAV's in 3D space within aspherical error probable of 1000 feet when the distance is 50 nauticalmiles or less. The coarse geolocation information may be transmittedusing frequency hopped spread spectrum waveforms to increase jamresistance.

The plurality of aerial vehicles may include one master aerial vehicle,and two secondary aerial vehicles each synchronized with the masteraerial vehicle using a stable rubidium reference oscillator on themaster aerial vehicle. The master aerial vehicle may be equipped with asynthetic aperture radar (SAR) or other equipment configured to producea terrain strip-map, and may be further configured to mensurate thestrip-map against a map database to provide fine position adjustmentsfor each of the plurality of aerial vehicles. Alternatively, each of theplurality of aerial vehicles may also be equipped with a SAR to provideitself with its own fine position adjustments. The terrain strip-map maybe mensurated against a level two digital terrain elevation map datasetto provide fine position adjustments for each of the plurality of aerialvehicles for a respective positional accuracy being within thirtymeters; or alternatively, the terrain strip-map may be mensuratedagainst a google earth map to provide fine position adjustments for eachof the plurality of aerial vehicles for a respective positional accuracybeing within three meters.

Each of the plurality of aerial vehicles may also be equipped with areceiver, and a transmitter configured to transmit a respective signalover an ISM band frequency that is coded with its latitude, longitude,and altitude, and modulated with non-random, non-repeating patterns, sothat a ground-based (or an air-based) computing device may perform timedifference of arrival measurements of the respective signals todetermine a location of the computing device. The location being soderived may be compared to a GPS derived location to determine of GPSspooling is occurring in the region.

BRIEF DESCRIPTION OF THE DRAWINGS

The description of the various example embodiments is explained inconjunction with appended drawings, in which:

FIG. 1 illustrates a first arrangement for deploying the systemdisclosed herein for providing highly jam-resistant and spoof-resistantlocation signals in support of an air or ground operation or other typeof operation.

FIG. 2A illustrates an isosceles triangular shape that may be used forpositioning of the air vehicle platforms deployed in the system of FIG.1.

FIG. 2B illustrates an equilateral triangular shape that may be used forpositioning of the air vehicle platforms deployed in the system of FIG.1.

FIG. 2C illustrates a scalene triangular shape that may be used forpositioning of the air vehicle platforms deployed in the system of FIG.1.

FIG. 2D illustrates use of four air vehicle platforms for the systemdisclosed in FIG. 1.

FIG. 3 illustrates use of the present invention for landing anexpeditionary force in a denied area, where a landing craft andpersonnel rely on the system for accurate navigation to designatedpoints of interest.

DETAILED DESCRIPTION OF THE INVENTION

As used throughout this specification, the word “may” is used in apermissive sense (i.e., meaning having the potential to), rather thanthe mandatory sense (i.e., meaning must). Similarly, the words“include”, “including”, and “includes” mean including but not limitedto.

The phrases “at least one”, “one or more”, and “and/or” are open-endedexpressions that are both conjunctive and disjunctive in operation. Forexample, each of the expressions “at least one of A, B and C”, “one ormore of A, B, and C”, and “A, B, and/or C” mean all of the followingpossible combinations: A alone; or B alone; or C alone; or A and Btogether; or A and C together; or B and C together; or A, B and Ctogether.

Also, all references (e.g., patents, published patent applications, andnon-patent literature) that are cited within this documents areincorporated herein in their entirety by reference.

Furthermore, the described features, advantages, and characteristics ofany particular embodiment disclosed herein, may be combined in anysuitable manner with any of the other embodiments disclosed herein.

FIG. 1 illustrates a first system and arrangement for providing highlyjam-resistant and spoof-resistant location signals in support of an air,sea, or ground operation. As shown in FIG. 1, three widely spaced,precisely located air vehicles may preferably be used. The air vehiclesmay be any suitable air vehicle, including, but not limited to aircraft(e.g., jet aircraft), helicopters, hot air balloons, unmanned aerialvehicles (UAVs), such as the Boeing A160T, “pseudolites” (i.e.,pseudo-satellites), stratollites and other advanced balloons such asthose made by World View, etc. In one embodiment the air vehicles usedmay exhibit a low radar cross-section. Any generic use of “UAV” as adescriptor for the air vehicle hereinafter is used merely forconvenience, and is not intended to limit the types of air vehicles thatmay be utilized in the system and method described herein. Also, incertain embodiments discussed hereinafter, the system and method may beutilized in conjunction with platforms other than air vehicles. However,merely to simplify the presentation, an exemplary discussion followsthat is made solely with reference to air vehicles, without intendingcertain embodiments to be so limited.

The air vehicles may preferably be flying at high altitudes above theground level (AGL). Flying at high altitudes may generally enable theair vehicles to operate free from attack, and the precise altitudeutilized may correspond to the available intelligence data for eachparticular mission. The altitude selection is particularly important forcertain missions, because the air vehicles may preferably operate in ahover mode to provide the most accurate position location information,and must avoid anti-aircraft artillery (AAA) gunfire to survive. Theenemy air defense radar capability may also be factored into placementof the UAVs, particularly its capability to detect low radarcross-section air targets that may be tens of miles distant.

For example, if intelligence data suggests that there are only 37 mmanti-aircraft guns in the area, which have a maximum effective altitudeof 15,000 feet, the air vehicle may be a tactical UAV hovering ataltitudes of up to 18,000 feet AGL, or may be a MALE drone (i.e., mediumaltitude, long endurance drone), as MALE drones may fly up to 30,000feet. If intelligence data indicates potentially hostile use of 85 mmAAA in the mission corridor, which have a maximum effective verticalfiring range of around 34,000 feet, a HALE drone (high altitude, longendurance drone) may be utilized, as HALE drones may fly at altitudeswell in excess of 30,000 feet. Therefore, air vehicle selection maycorrespond to the mission-specific environment. In general, an altitudeof at least 20,000 feet may be preferable to place the three spacedapart air vehicles out of both audible (hearing) range and sightingrange.

Moreover, in various other embodiments, the apparatus forming the systemdisclosed herein is not limited to use in the described scenarioinvolving air vehicles, as the herein disclosed apparatus may also becovertly positioned on the rooftops of buildings, or on buoys anchoredoff-shore, or on small raft or other watercraft anchored far away. Inone embodiment miniature drones, each of which may weigh only ounces andwhich have the ability to navigate autonomously, can be used and bedirected to land at concealed locations on rooftops. Note that atradeoff with such an embodiment may be greater likelihood of multipathand potentially lower Effective Isotropic Radiated Power (EIRP) due tolimited prime power (i.e., reliance on battery power) and a very limitedantenna size. Even submersible vehicles with suitable antenna(s) may beutilized in certain embodiments. Use of the lowest technology and leastexpensive platform type may be preferred, to minimize the potential lossin case of a failure or an unforeseen attack, as long as the particularplatform selected may perform adequately to satisfy the anticipatedmission criteria/conditions.

In an embodiment where air vehicles are utilized, an added benefit ofpositioning them at a high altitude is that it typically places the airvehicles above adverse weather, and away from winds that may causebuffeting that would tend to displace each of the vehicles from itsdesignated stationary position. Such temporal displacements may affectaccuracy or may reduce on-station time. The high altitudes alsogenerally serves to maximize covertness of the three UAV's. In oneembodiment, the platforms may be located at a minimum altitude of 20,000feet AGL to ensure that they cannot be detected either visually byhostile forces on the ground, or through an audible sound. The higheraltitudes may also benefit the suppression of multipath. Multipathsuppression (and jam resistance) will also be aided by prudent waveformselection such as wideband direct sequence spread spectrum (DSSS) forthe Time Difference of Arrival (TDOA) measurements discussedhereinafter.

The three stationary platforms may preferably be positioned to be widelyspaced apart, and may, in one embodiment, be spaced between five milesto fifty miles apart, and may be uniformly spaced apart. In otherembodiments, other spacing distances that support proper signaltransmission/reception may be used. The three platforms may preferablyalso be positioned at the corners of an imaginary triangle T1. In oneembodiment, the three platforms may be placed at the corners of animaginary isosceles triangle (FIG. 2A), which may be an equilateralisosceles triangle (FIG. 2B).

The three stationary air vehicle platforms need to be synchronized,because the process of geolocation for the air, sea, or ground operationrelies on time difference of arrival measurements of the three UAVsignals by the user interface (e.g., computing device) on the ground, asdiscussed hereinafter. Carrier frequencies must be identical as well asthe modulations on each of the three carriers. In order to time-alignthe received signals at the user's location, the modulation of thecarriers must contain non-random, non-repeating patterns and thesepatterns must be synchronized in time to support unique time-alignmentsat the user interface. Therefore, in one embodiment, one of the airvehicle platforms is designated as the “master” (e.g., AV₁), as theother two air vehicle platforms (e.g., AV₂ and AV₃) each have theirfrequency and timing synchronized to that of the master platform AV₁.

The master platform AV₁ may be equipped with a highly stable Rubidiumreference oscillator AV_(RU) that provides precise frequency and timinginformation for the trio of platforms. Also, each of the three airvehicles AV₁, AV₂, and AV₃ may be equipped with phase-stable,delay-matched receivers and transmitters (TR1, TR2, and TR3) that mayoperate in the UHF/microwave region (e.g., roughly 425 MHz to 2.4 GHz).These transmitters may make use of circuitry to maximize re-use andreduce SWaP-C, although the process of transmitting coordinateinformation does not require phase stability and/or control of delay, asrequired for transmitting of the time difference of arrival signalsdiscussed hereinafter, because any perturbations and/or differences mayresult in synchronization errors and may manifest as geolocationmeasurement errors to the mission participants in the air, sea, orground operation.

The rubidium reference oscillator on the master platform generates ahighly stable 6.834,682,612 GHz microwave reference frequency with longterm stability of 5×10⁻¹². All commercial rubidium frequency standardsoperate by disciplining a crystal oscillator to the hyperfine transitionat 6.834,682,612 GHz in rubidium. This reference is input to a divide by3 frequency divider to produce a 2.278227537 GHz reference signal whichis then amplified and distributed wirelessly to each of the twosynchronized UAVs. Translation to this S-Band frequency obviates theneed for separate, dedicated, antenna and receiver electronics todistribute the reference when the system operating frequency is in the2.3 to 2.4 GHz range. The close proximity of reference and operatingfrequencies permits hardware sharing which in turn reduces overallhardware SWaP-C. To derive a rubidium disciplined 10 MHz reference, the2.278227537 GHz signal is received by the two synchronized UAVs,amplified, filtered, and further conditioned to serve as the systemclock reference for a direct digital synthesizer (DDS). The output ofthe DDS is a rubidium-disciplined 10 MHz sine wave reference whichserves as the timing reference for all timing-critical operations.

In addition to the three platforms (e.g., air vehicles AV₁, AV₂, andAV₃) being positioned in the triangular pattern, a “mother” air vehicleplatform AV_(M) may be utilized in the system. The mother platformAV_(M) may be any of the above-described platform types. In anotherembodiment the mother air vehicle platform AV_(M) may be an airbornewarning and control aircraft (AWACS/AEWCS), including, but not limitedto the Northrop Grumman E-2 Hawkeye, the Boeing E3 Sentry, etc. In yetanother embodiment the mother platform AV_(M) may be a suitably equippedhelicopter (i.e. carrying a suitable radar system), including, but notlimited to the Navy's MH-60R antisubmarine warfare (ASW) helicopter. Thearmy's STARLite Radar-equipped MQ-IC Gray Eagle may also be used. Themother air vehicle AV_(M) may be positioned at a safe stand-off distanceaway from the other three stationary air vehicles, and typically (i.e.,in one embodiment) it may reach a maximum altitude of 10,000 feet. Otheraltitudes may also be utilized. Also, typically (i.e., in oneembodiment) the three triangular-positioned platforms AV₁/AV₂/AV₃ may bepositioned at a higher altitude than that of the mother platform. Inother embodiments, the three triangular-positioned platforms AV₁/AV₂/AV₃may be positioned at a lower altitude than that of the mother platform,or at the same altitude.

The mother air vehicle AV₁ may preferably be a significant distance awayfrom the adverse territory (i.e., away from the GPS-denied/GPS-spoofedterritory), being positioned where the GPS signals are uncompromised. Inone embodiment the mother air vehicle AV₁ may be a distance away fromthe three air vehicles AV₁/AV₂/AV₃ that may be 10 times the spacingbetween the trio of air vehicles. In another embodiment the mother airvehicle AV_(M) may be a distance away from the three air vehiclesAV₁/AV₂/AV₃ that may be 20 times the spacing between the trio of airvehicles. In yet another embodiment the mother air vehicle AV_(M); maybe a distance away from the three air vehicles AV₁/AV₂/AV₃ that may beat least 50 times the spacing between the trio of air vehicles. In afurther embodiment the mother air vehicle AV_(M) may be a distance awayfrom the three air vehicles AV₁/AV₂/AV₃ that may be at least 100 timesthe spacing between the trio of air vehicles. Thus, the mother airvehicle AV_(M) may be positioned many hundreds of miles away from thethree triangular-positioned air vehicles AV₁/AV₂/AV₃.

Being so positioned the mother air vehicle AV_(M) may be able to receiveunperturbed GPS signals and in turn may transmit coarse geolocationinformation via an X-Band link to each of the three stationary airvehicles AV₁/AV₂/AV₃, and may thereby command and/or direct them toapproximate locations in 3D space, as follows. The radar used on themother air vehicle AV_(M) to geo-locate the three stationary airvehicles AV₁/AV₂/AV₃ in space is preferably a wide instantaneousbandwidth, high resolution radar system, which facilitates highlyaccurate distance measurement of the UAVs. This radar also has theability to locate the targets azimuthally to within fractions of adegree by virtue of its physically large antenna and beam splittingfunction. (Note the elevation of the three air vehicles AV₁/AV₂/AV₃ isdetermined by highly accurate barometric sensors). This radar used onthe mother air vehicle AV_(M) may utilize pulse compression, highaverage power X Band signals (i.e., average power being 200-300 watts).Therefore, two important attributes of this radar system are itsexcellent bearing accuracy of 0.2 degrees rms and an RMS rangemeasurement accuracy of less than two hundred feet. Thus, in oneembodiment, the radar may be capable of coarsely locating the threestationary air vehicles AV₁/AV₂/AV₃ in 3D space within a Spherical ErrorProbable (SEP) of 1000 feet at stand-off distances of up to 50 nauticalmiles from the mother air vehicle AV_(M). The radar system used on themother air vehicle AV_(M) may also incorporate a communications functionto allow it to “talk” to the three stationary air vehicles to transmitsuch position information, and it may therefore be equipped with acommunications channel to transmit coordinate information at low datarates to the UAVs. A small portion (e.g., <5%) of the radar timeline maybe appropriated and assigned to support this communications function.The mother air vehicle AV_(M) may use this communication function of theradar to transmit the measured initial position (i.e., latitude andlongitude data) to each of the UAVs, and also the coordinates (latitudeand longitude) for the final required/designated positions. Thiscommunication feature of the radar is an enhancement to Applicant'sMH-60R radar, and may be the subject of a separate application.

However, for the range and azimuth measurements to be accurate, advanceknowledge of both the altitude of the mother air vehicle AV_(M) and thealtitudes of the three stationary air vehicles AV₁/AV₂/AV₃ is required.The mother air vehicle AV_(M) may have an integral wideband microwavealtimeter to accurately determine its altitude. The altitude of themaster air vehicle AV₁ may also be determined via its own on-boardwideband microwave altimeter. In one embodiment, the altitude of thethree stationary air vehicles AV₁/AV₂/AV₃ will not be transmitted backto the mother ship, and so the altitude of the master air vehicle AV₁(and therefore all three air vehicle) may be preset (e.g., to be 20,000ft.).

Furthermore, to prevent the coarse geolocation information transmittedto the 3 UAVs from the mother air ship AV_(M) via the X Band link frombecoming compromised in the same way that the GPS signals may bespoofed, the X Band link will be configured to exhibit very good jamresistance through the use of a highly directional antenna that uses avery narrow beam width (e.g., on the order of 1 degree), and usesextremely low side lobe levels (e.g., <40 dB). Frequency hopped spreadspectrum waveforms may also be employed to increase its jam resistance.

In one embodiment, to improve latitude and longitude measurementaccuracy even further, the coarsely-positioned master air vehicle AV₁may be configured for terrain and feature mapping using an onboard highresolution synthetic aperture radar (SAR) AV_(SAR) that is capable ofgenerating a SAR strip-map of a scene, and is further configured formensurating the map against a map database. (Note—in general,mensuration is the process of measuring a feature or location on theearth to determine an absolute latitude, longitude, and elevation). TheSAR strip-map may be mensurated with a terrain reference, such as LevelII Digital Terrain Elevation Data (DTED) “maps.” which consist ofdigital datasets containing a matrix of terrain elevation values.(Note—Level II DTED has post spacing of approximately 30 meters—See.U.S. Military Specification Digital Terrain Elevation Data (DTED)MIL-PRF-89020B). Therefore, this only yields a positional accuracy foreach of the three UAVs of only 30 meters. Note, the reference imagerymay also be digital precision database (DPPDB) imagery supplied by theNational Geospatial Intelligence Agency (NGA). Registration with ageospatial database provides capability to refine position informationand tie the derived information to a validated targeting source using aproprietary log-polar transform (modLPT) registration algorithm.

The synthetic aperture radar AV_(SAR) of the master air vehicle AV₁ mayalso be configured for mensurating the SAR strip-map from the onboardSAR radar against Google Earth maps of the disadvantaged territory thatmay be stored in the database, which requires knowledge of a SAR imagedepression angle (or grazing angle) and platform elevation, both ofwhich are available. This permits the coarsely-positioned master airvehicle AV₁ to thereafter constantly provide finer adjustments to itsposition. Accurate differential repositioning of the other two platformsmay also be subsequently done. This may result in each of the threeplatforms AV₁/AV₂/AV₃ being located within several meters of a desiredtrue position, even when at high altitudes. (Note, in other embodiments,each of the two synchronized platforms AV₂/AV₃ may alternatively beconfigured to be able to make its own fine position adjustments).

The master air vehicle AV₁ transmits respective initial locations andfinal desired locations to the synchronized air vehicles AV₂/AV₃. Onboard each of the three platforms AV₁/AV₂/AV₃ may be a highly accuratemilitary grade inertial Navigation System (INS), capable of maintainingtrue position to within 1 nautical mile over a 24 hour period withoutupdates. During the mensuration process the three UAVs may be in closeproximity (i.e., within one to two meters) of one another. To programtheir initial locations, the INS on-board each of the two synchronizedUAVs AV₂/AV₃ is initialized with the measured latitude/longitude datafrom the master air vehicle AV₁, which data may be transmitted to thetwo synchronized UAVs immediately upon the master air vehicle AV₁ havingdetermined its own true position based on the mensuration process.Waypoints (i.e., destination coordinates in terms of latitude andlongitude, and perhaps altitude as well) that represent the finaldesired locations for correct system operation and desired groundcoverage may then be programmed in this manner.

After each of the three triangular-positioned platforms AV₁/AV₂/AV₃ areprecisely positioned within several meters of its desired true position,Time Difference of Arrival (TDOA) measurements may be made using signalsrespectively transmitted by the three UAV's. The TDOA measurements maybe made using a tactical computing device 30 possessed by the missionparticipant(s) 20 who must directly interact in the GPS comprisedterritory, and who require accurate position location information (PLI)within the hostile theatre. The mission participant(s) 20 may include,but is/are not limited to, a fifth air vehicle, a surface vehicle, anamphibious vehicle, an isolated Special Forces soldier, a team ofsoldiers, etc.

The ruggedized tactical computing device (TCD) is a potentially powerfultool that may not only support geolocation in hostile areas, but whichcan also ferret out GPS spoofing. The TCD may also function to “call in”air support and/or supply targeting information via the UAV(s), using anapplication, including, but not limited to, “MAFIA” (Maneuver Aviationand Fires Integrated Application), which was developed by the U.S. ArmyAviation and Missile Research. Development and Engineering Center(AMRDEC). The platforms may thus also be configured to form abeyond-line-of-sight communication link to either another airborneplatform for relay to the ground, or to other overhead assets. The threeUAVs can serve as a wideband “hot spot” enabling disparately locatedSOFs to communicate with and transfer data between one another overdistances that far exceed the capabilities of ground based hot spots.The very high EIRP and wide band (approaching 84 MHz) may permit highdata rate transfer of information.

In one embodiment, the computing device 30 that may be provisioned withthe mission participant(s) 20 may be a variant of a ruggedized iPhonewith GPS, which may also be configured to receive the signals from thethree UAV's (AV₁/AV₂/AV₃) and may perform the required computations viaan application load. Each of the three UAVs may transmit a unique codedreference signal (SIG1, SIG2, SIG3) to allow the ruggedized computingdevice 30 to separate one signal from another, and, knowing the signal'sorigin, it may compute the time difference of arrival. Each of thereference signals (SIG1, SIG2, SIG3) may be coded to include itslatitude, longitude, and altitude, which may be refreshed at a rate of 5Hz. The ruggedized computing device 30 may also include a barometricsensor configured to provide elevation information that may be neededfor the geolocation process. Altitude may also be obtained from awideband S-band radar altimeter. Coding methods that may be used for thereference signals may include, but are not limited to, Direct Sequence(DS/SS) coding, and Frequency Hopping (FH/SS) coding.

The signals from the three UAVs may be covertly transmitted oversegments of the ISM band (e.g., 2.4 GHZ), which may be used for theWi-Fi connection of the ruggedized computing device 30. For additionalcovertness it is also possible in one embodiment, with the appropriatepermissions, to “bury” the UAV signals in any of the cellular bandsbelow 2.4 GHz. For example, a country friendly to the west (e.g., a NATOcountry such as Estonia) may be targeted by a hostile invading force andthe invading force may jam the GPS over wide areas of the country. Inthis instance, without any prior agreement with the invaded nation.Estonia may object to NATO usurping and/or interfering with its cellularnetwork. However, if DSSS signals are utilized, they could take on aclandestine character and not interfere with normal cellularcommunications.

Providing the mission participants 20 with the ability to process bothGPS signals and the UAV signals simultaneously allows the SpecialOperations Forces (SOF) soldier/team, to rapidly determine if spoofingis occurring. Spoofing can be determined by comparing thelatitude/longitude data derived from the GPS signals, with the same dataderived from the UAV signals (SIG1, SIG2, SIG3), and the existence ofsignificant differences would indicate that spoofing is occurring. Whenspoofing is occurring, reliance on GPS signals would necessarily beavoided, and navigation would principally be through use of the positiondata obtained from the UAV signals, which may constantly provideaccurate geolocations for the mission participants. Typical GPS signalconditions are such that outdoor signal levels of less than −130 dBm arecommonly encountered, and as many as five satellites may be required.Therefore, the much higher signal levels used in the system andarrangement disclosed herein and the use of fewer reference sources mayenable this signal acquisition and TDOA measurements to ordinarily be avastly simpler undertaking, and may provide substantially improvedaccuracy and reliability for the geolocation information therebyobtained by the mission participants.

With respect to the geolocations thereby provided by the TDOAmeasurements from the UAV signals, the geolocation accuracy may beimproved (i.e., may be maximized) when the mission participants 20 areroughly located at the normal projection TIC of the geometric center ofan equilateral isosceles triangle (see FIG. 1) that is formed by thethree air vehicles AV₁/AV₂/AV₃.

As noted above, in one embodiment the distance between the UAVs may bebetween 5-50 miles. In another embodiment the UAVs may be positioned tobe spaced at least 25 miles apart, which spacing may be uniform. Otherdistances for spacing may also be used that would support proper signaltransmission/reception. Varying this spacing (i.e., increasing ordecreasing the distance between UAVs) may serve to improve and/ordiminish certain aspect of the system's performance. Closer spacingbetween the UAVs may in certain cases diminish the benefits to themission participants because the area in square miles for the theatre ofground operations concomitantly becomes more limited. A five-milespacing between three UAVs positioned at the theoretical corners of anequilateral isosceles triangular provides roughly 10.8 square miles ofprojected ground area coverage, while a ten-mile spacing provides 43.3square miles of projected area. For greatest accuracy (e.g., to minimizeGeometric Dilution of Precision), the mission participants 20 should bepositioned within the boundaries of the triangle projected on the ground(i.e., be in its “shadow”). Geospatial errors can be 10× greater at aposition on a boundary of the triangle, as compared to geospatial errorsat the center of the triangle. With an equilateral triangular for theUAV spacing, as shown for example in FIG. 1, acceptable performance mayreadily be expected within a circle of radius 3.53 miles having an areaapproximately equal to 40 square miles, which should be adequate formost expeditionary missions

Increasing the spacing between the master and synchronized UAVs mayresult in degraded location accuracy for the synchronized UAVs (withrespect to latitude and longitude, not elevation). The SAR radarAV_(SAR) on the master platform AV₁, operating in its high bearingaccuracy mono-pulse mode, is responsible for locating the synchronizedUAVs accurately in azimuth space. Under high signal-to-noise (SNR)conditions (e.g., >14 dB), the accuracy degrades linearly with thedistance, d, between master and slave UAV's. At greater distances, d,where SNR is progressively less, the azimuth accuracy will varyapproximately in accordance with 1/d^(3/2). The distance measurementaccuracy will however vary as 1/d^(1/2).

The mission participant(s) on the ground is affected differently:increased spacing has the benefit of increasing the measurement baselineand, as in any interferometric (TDOA) measurement system, increasing thespacing (or baseline) increases measurement accuracy. Of course, as withany generalization, there are limits to its validity. RMS accuracy willvary as 1/d^(1/2) due to variation in SNR. In the design of the system,high EIRP conditions may be maintained so that SNR never drops below 20dB and impacts to accuracy are minimized.

It should also be noted that the route and ultimate target location inthe hostile territory for the mission participants 20 is more often thannot fixed is 3D space. Therefore, in order for the mission participants20 to be generally centered with respect to the triangle T1 formed bythe three air vehicles AV₁/AV₂/AV₃ (or at least be underlying it), theUAVs must be positioned with respect to that route and the target, andthe triangle thereby formed may in certain cases need to be repositionedthroughout the course of the mission. For example, if the missionparticipants must traverse a long distance (e.g., 27 miles) within ahostile GPS-spoofed territory, and if the UAVs are spaced 5 miles apart,the triangular positioned UAVs (AV₁/AV₂/AV₃) may preferably driftuniformly in 3D space in correspondence with the progress of the missionparticipants along the 27 mile route (see e.g., FIG. 2D), to maintainthe provision of optimal positional accuracy. Alternatively, as shown inFIG. 2D, an additional UAV AV₄ may be timely commanded into position toform a second triangle, to continue to provide accurate positionalinformation to the mission participant 20, rather than requiring theUAVs AV₁/AV₂/AV₃ to drift uniformly. Also, all four UAVs may be utilizedfrom the outset, and may form a parallelogram shape. Other geometricshapes may also be used. Also, a plurality of UAVs positioned to form anodd shaped array may be utilized, so that no movement may be necessaryto provide positional information throughout the course of an extendedoperation. Also, additional platforms can further benefit geolocationaccuracy, particularly in areas away from sea level and in mountainousareas.

In certain cases, particularly in mountainous terrain, use of only thesignals from the three air vehicles AV₁/AV₂/AV₃ may not be sufficient tolocate the mission participants in 3D space; however, in one embodimentthe mission participants 20 may also measure his/her altitude (e.g., viaan accurate barometric sensor in the hand-held device that determinespressure altitude, which may be converted to geometric altitude, and maypreferably be accurate to within 10 to 20 meters). The altitudemeasurement may be done automatically, and without any operatorinvolvement.

The centralized location for the mission participants 20 with respect tothe normal projection TIC of the geometric center of the triangleminimizes the Geometrical Dilution of Precision (GDOP), and results inthe lowest positioning error. For that reason, the optimal triangularshape that may be formed by the three UAVs may preferably be anequilateral triangle, which is a particular type of isosceles trianglein which all three sides (rather than just two sides) of the triangleare congruent. Note a scalene triangle as shown in FIG. 2C may also beused, but the sides of any theoretical triangle used for positioning theUAVs would desirably be close in length. If the triangle utilizedbecomes quite exaggerated, the accuracy will be degraded (e.g., if usingan obtuse triangle with the one obtuse angle being much greater thanninety degrees; or using an acute triangle with two of the angles eachapproaching ninety degrees such that it has one very short side and twomuch longer sides). The GDOP and UAV placement accuracies will both tendto degrade.

Minimizing GDOP is a goal, and its effect on PLI accuracy is illustratedin the following example.

With a spherical error probable of, for example, 100 feet, and a GDOPthat may approach 10 when the user straddles one of the sides of theimaginary triangle, the positioning error could approach 1000 feet. The“volume” of the Spherical Error Probable is a function of the accuracyof the rms elevation, latitude, and longitude measurements. For accurateelevation data the system will depend on the radar altimeter of theUAVs, since the EGI (GPS+INS) will be disabled due to the disadvantagedenvironment. For altimeter bandwidths approaching 200 MHz the elevationmeasurement will be accurate to within three feet, not the limiting casefor SEP. The latitude and longitude measurements may, however, limitSEP. Initial accuracy may be a function of the range measurementaccuracy and azimuth pointing accuracy of the X-Band radar on board themother UAV AV₁, which is 200 feet and 1000 ft. rms, maximum,respectively. Initial development of this state of the art radar wasaccomplished by Telephonics Corp. in the 1990's. The X-Band radar is theradar of choice for a number of demanding military missions. Thelatitude and longitude measurement accuracies assumed are consistentwith the bandwidth and azimuth half power beam-width (HPBW) propertiesof this radar and the mode design that we will utilize.

As noted herein above, another source of error is the relative motionbetween the UAVs AV₁/AV₂/AV₃ due to buffeting caused by wind gusts andother disturbances. By implementing the master/synchronized arrangementand monitoring and recording displacement of the platforms using theplatform INS, the effects of platform motion on geolocation accuracy canbe suppressed. However, to maintain position accuracy information, themaster UAV location coordinates, as established through the mensurationprocess, may preferably be maintained by exercising platform controls asneeded

The following is a series of steps for geo-locating mission participantsin a GPS denied area, some or all of which steps may be used in variousdifferent embodiments:

-   -   1. Three air vehicles that are intended to occupy a stationary        position in support of a covert mission leave a base of        operation and hover in close proximity to each other near the        location of the covert operation, which stationary position is        known by a mother air vehicle.    -   2. One of the three air vehicles is designated the master UAV        and will have its own on-board wideband microwave altimeter that        is used to position the three air vehicles at a preset altitude,        which is known by the mother Air Vehicle (e.g., 20,000 ft.).    -   3. The master air vehicle synchronizes each of the three air        vehicles (i.e., provides precise frequency and timing        information therebetween) using a stable rubidium reference        oscillator on the master air vehicle, and using phase-stable,        delay-matched receivers and transmitters on each of the three        air vehicles.    -   4. The mother air vehicle leaves a base of operation (e.g., a        navy destroyer, aircraft carrier, land base, etc.) and navigates        to a designated geo-position (perhaps hundreds of miles away        from the location of the covert operation and from the position        three air vehicles), where it may acquire unperturbed GPS        signals and obtain position info.    -   5. The mother air vehicle positions itself at a predetermined        altitude (e.g., 10,000′ AGL) using an onboard wideband microwave        altimeter.    -   6. The mother air vehicle geo-locates the three closely        positioned air vehicles in space using an onboard wide        instantaneous bandwidth, high resolution radar, which        facilitates highly accurate distance measurement of the UAVs.    -   7. The mother air vehicle radar transmits two locations to the        master air vehicle via an X band link with a very narrow beam        width—        -   i) the measured initial position (latitude and longitude) of            the three air vehicles; and        -   ii) the final required spaced-apart positions for each of            the three air vehicles, which may be spaced apart positions            that form a triangular shape.    -   8. The three air vehicles each maneuver from the closely spaced        initial positions to the respective required coarse        geo-positions.    -   9. The master air vehicle uses an onboard synthetic aperture        radar to produce a terrain strip-map and mensurates the terrain        strip-map against a map database to acquire fine position        adjustments for each of the three air vehicles.    -   10. The master air vehicle transmits the fine position        adjustments needed to the other two air vehicles, which are        programmed into an INS system used on each of the three air        vehicles.    -   11. The three air vehicles each maneuver to be stationary at the        respective fine-adjusted positions to form a triangular shape.    -   12. Each of the three UAVs transmit a unique reference signal        that is coded to include its latitude, longitude, and altitude.    -   13. The covert ground/air/sea mission forces use a barometric        sensor to determine pressure altitude, which is converted to        geometric altitude above mean sea level by a computing device.    -   14. The covert ground/air/sea mission forces use the computing        device to make time difference of arrival (TDOA) measurements of        the coded signals in combination with the acquired geometric        altitude for accurately determining their geo-location.    -   15. The accurate geo-location acquired from the TDOA        measurements may be compared with a GPS-derived geo-location of        the mission participants to determine if GPS spoofing is        occurring.    -   16. The mission proceeds with the mission participants        preferably remaining within the ground projection of the        triangle formed by the three air vehicles to ensure greater        positional accuracy.

While illustrative implementations of one or more embodiments of thepresent invention are provided hereinabove, those skilled in the art andhaving the benefit of the present disclosure will appreciate thatfurther embodiments may be implemented with various changes within thescope of the present invention. Other modifications, substitutions,omissions and changes may be made in the design, size, materials used orproportions, operating conditions, assembly sequence, or arrangement orpositioning of elements and members of the exemplary embodiments withoutdeparting from the spirit of this invention.

Accordingly, the breadth and scope of the present disclosure should notbe limited by any of the above-described example embodiments, but shouldbe defined only in accordance with the following claims and theirequivalents.

What is claimed is:
 1. A method for determining if spoofing of globalpositioning system (GPS) signals is occurring in a region, said methodcomprising: positioning first, second, and third aerial vehicles at afirst altitude and at a stationary position in proximity to the region;synchronizing the first, second, and third aerial vehicles with eachother; positioning a mother aerial vehicle a distance away from thefirst, second, and third aerial vehicles at a second altitude;transmitting coarse geolocation information by the mother aerial vehicleto each of the first, second, and third aerial vehicles, therebydirecting the first, second, and third aerial vehicles to respectivecoarse positions above a corresponding ground location in the region;producing a terrain strip-map by one or more of the first, second, andthird aerial vehicles and mensurating the terrain strip-map against amap database; providing fine position adjustments to each of the first,second, and third aerial vehicles, based on said mensurated terrainstrip map; transmitting a respective coded signal by each of the first,second, and third aerial vehicles; determining a location in the regionby comparing time difference of arrival (TDOA) measurements of therespective coded signal at the location; and determining if spoofing isoccurring in the region by comparing said determined location with aGPS-derived location.
 2. The method according to claim 1, furthercomprising: coding each said respective coded signal with a latitude, alongitude, and an altitude of the respective aerial vehicle; andmodulating each said respective coded signal with non-random,non-repeating patterns.
 3. The method according to claim 2, furthercomprising: obtaining a mean sea level altitude; and determining thelocation in the region using the mean sea level altitude and the TDOAmeasurements of the unique coded reference signals.
 4. The methodaccording to claim 3, further comprising: synchronizing the first,second, and third aerial vehicles with the master aerial vehicle usingfrequency and timing data from a stable rubidium reference oscillator.5. The method according to claim 4, further comprising: synchronizingthe first, second, and third aerial vehicles with the master aerialvehicle by operating the stable rubidium reference oscillator in therange of 425 MHz to 2.4 GHz.
 6. The method according to claim 5, furthercomprising: transmitting each said coded signal over an ISM bandfrequency.
 7. The method according to claim 6, further comprising:refreshing each said respective coded signal at a rate of 5 Hz.
 8. Themethod according to claim 7, further comprising: mensurating thestrip-map against one or more of: a level two digital terrain elevationmap dataset; and a google earth map by said master aerial vehicle. 9.The method according to claim 8, further comprising: transmitting thecoarse geolocation information from the mother aerial vehicle to each ofthe first, second, and third aerial vehicles via an X-Band link.
 10. Themethod according to claim 9, further comprising: transmitting the coarsegeolocation information from the mother aerial vehicle to each of thefirst, second, and third aerial vehicles using a pulse compression,high-power X Band radar with a directional antenna, using a beam widthof about one degree with sidelobe levels of less than 40 dB.
 11. Themethod according to claim 10, further comprising: increasing jamresistance by transmitting the coarse geolocation information with thedirectional antenna using frequency hopping spread spectrum waveforms.12. The method according to claim 11, further comprising: positioningthe first, second, and third aerial vehicles at a higher altitude thanthe mother aerial vehicle.
 13. The method according to claim 12, furthercomprising: spacing the first, second, and third aerial vehicles betweenfive miles and ten miles apart.
 14. The method according to claim 13,further comprising: positioning the first, second, and third aerialvehicles at an altitude being above a range capability of anti-aircraftartillery gunfire.
 15. The method according to claim 14, furthercomprising: positioning the mother vehicle at an altitude being lessthan 10,000 feet AGL.
 16. A method of providing jam-resistant andspoof-resistant position information to a mission participant in adisadvantaged signal environment at a covert location, said methodcomprising: positioning each of a first aerial vehicle, a second aerialvehicle, and a master aerial vehicle at a preset altitude and at astationary position in proximity to the covert location; positioning amother aerial vehicle at a predetermined altitude and at a distance awayfrom the disadvantaged signal environment of the covert location;synchronizing the first aerial vehicle, the second aerial vehicle, andthe master aerial vehicle using frequency and timing information;measuring an initial position of the first aerial vehicle, the secondaerial vehicle, and the master aerial vehicle, by the mother aerialvehicle, using an onboard high resolution radar; transmitting twolocations from the mother aerial vehicle to the master aerial vehicle,via an X band link with a very narrow beam width and extremely low sidelobe levels, including: transmitting said measured initial position, andtransmitting respective spaced-apart positions above the covert locationfor each of the first aerial vehicle, the second aerial vehicle, and themaster aerial vehicle; maneuvering the first aerial vehicle, the secondaerial vehicle, and the master aerial vehicle to the respectivespaced-apart positions; acquiring respective fine position adjustmentsfor each of the first aerial vehicle, the second aerial vehicle, and themaster aerial vehicle, by producing a terrain strip-map using asynthetic aperture radar on the master air vehicle, and mensurating theterrain strip-map against a map database; transmitting the respectivefine position adjustments from the master aerial vehicle to the firstaerial vehicle and the second aerial vehicle; maneuvering the firstaerial vehicle, the second aerial vehicle, and the master aerial vehicleto its respective fine-adjusted position; coding, by each of the firstaerial vehicle, the second aerial vehicle, and the master aerialvehicle, a unique reference signal including its latitude, longitude,and altitude transmitting, by each of the first aerial vehicle, thesecond aerial vehicle, and the master aerial vehicle, its unique codedreference signal; determining a geo-location of the mission participantby performing time difference of arrival (TDOA) measurements of theunique coded reference signals, using a computing device.
 17. The methodaccording to claim 16, further comprising: obtaining a mean sea levelaltitude of the mission participant by determining a pressure altitudeusing a barometric sensor, and converting said determined pressurealtitude to the mean sea level altitude a using the computing device;determining the geo-location of the mission participant using the timedifference of arrival (TDOA) measurements of the unique coded referencesignals in combination with the obtained mean sea level altitude. 18.The method according to claim 17, further comprising: coding the uniquereference signal using direct sequence coding.
 19. The method accordingto claim 17, further comprising: coding the unique reference signalusing frequency hopping (FH/SS) coding.
 20. The method according toclaim 16, further comprising: determining if spoofing of globalpositioning system (GPS) signals is occurring at the covert location bycomparing said determined geo-location from the TDOA measurements with aGPS-derived location.